In one example of local area network, a fiber extends around a loop or ring. Light is transmitted from a central office around the loop from a modulated light emitting source. Depending on the activity around the loop, light is received back from the loop at a photodetector in the central office.
That activity may take the form of a variety of passive or active fiber optic devices which are inserted into the line at a number of discrete positions depending on the particular functional requirement at each position.
The functional requirements are for example a fiber optic tap at which optical signals can be both diverted from and inserted into the fiber optic channel, or a subsidiary loop providing an extension to the network, or a wavelength division multiplexer or demultiplexer for inserting or extracting light of a particular wavelength into or from the loop fiber, or finally, an active module such as a repeater to boost the signal within the loop fiber. Most known structural implementations of these functional requirements are characterized by a metal or plastics package including a container part through the wall of which extend one or more optical fibers in sealed feedthrough arrangements. Typically the package has a cover which is screwed, glued or soldered to the container part into which optical and electro-optical elements are fixed.
These known device packages cannot easily be modified in order to substitute functions. A modular arrangement is now proposed which enables functional changes at an optical fiber access node to be made relatively quickly.